Application addon using bpy.data.scenes causes: AttributeError: '_RestrictData' object has no attribute 'scenes'
You shouldn't access or change properties before the add-on is properly loaded. The unit settings can be changed for example in an operator as can be seen below:

bl_info = {
    "name": "Make Imperial",
    "author": "Robert Guetzkow",
    "version": (1, 0),
    "blender": (2, 80, 0),
    "location": "Dummy panel in View3D",
    "description": "Changes the unit settings to imperial",
    "warning": "",
    "wiki_url": "",
    "category": "3D View"}

import bpy


class EXAMPLE_OT_make_imperial(bpy.types.Operator):
    bl_idname = "example.make_imperial"
    bl_label = "Set unit settings to imperial"
    bl_options = {'REGISTER', 'UNDO'}

    def execute(self, context):
        bpy.context.scene.unit_settings.system = 'IMPERIAL'
        return {'FINISHED'}


class EXAMPLE_PT_panel(bpy.types.Panel):
    bl_label = "My own addon"
    bl_category = "Name of your tab"
    bl_space_type = "VIEW_3D"
    bl_region_type = "UI"

    def draw(self, context):
        layout = self.layout
        layout.operator(EXAMPLE_OT_make_imperial.bl_idname)


classes = (EXAMPLE_PT_panel, EXAMPLE_OT_make_imperial)


def register():
    for cls in classes:
        bpy.utils.register_class(cls)


def unregister():
    for cls in classes:
        bpy.utils.unregister_class(cls)


if __name__ == "__main__":
    register()